Organizations such as on-line retailers, Internet service providers, search providers, financial institutions, universities, and other computing-intensive organizations often conduct computer operations from large scale computing facilities. Such computing facilities house and accommodate a large amount of server, network, and computer equipment to process, store, and exchange data as needed to carry out an organization's operations. Typically, a computer room of a computing facility includes many server racks. Each server rack, in turn, includes many servers and associated computer equipment.
Because the computer room of a computing facility may contain a large number of servers, a large amount of electrical power may be required to operate the facility. In addition, the electrical power is distributed to a large number of locations spread throughout the computer room (e.g., many racks spaced from one another, and many servers in each rack). Usually, a facility receives a power feed at a relatively high voltage. This power feed is stepped down to a lower voltage (e.g., 208V). A network of cabling, bus bars, power connectors, and power distribution units, is used to deliver the power at the lower voltage to numerous specific components in the facility.
Primary power systems for computer systems in operation typically need to be maintained or reconfigured from time to time. Some data centers, for example, have “single threaded” distribution via the electrical power supply to the floor and/or to the rack, and in which maintenance can only be performed when the components using power in the data center, such as servers, are shut-off. The down-time associated with maintenance and reconfiguration of primary power systems in a data center may result in a significant loss in computing resources. In some critical systems such as hospital equipment and security systems, down-time may result in significant disruption and, in some cases, adversely affect health and safety.
Some systems include dual power servers that provide redundant power for computing equipment. In some systems, an automatic transfer switch (“ATS”) provides switching from a primary power system to a secondary (e.g., back-up) power system. In a typical system, the automatic transfer switch automatically switches the computing equipment to the secondary system upon detecting a fault in the primary power. To maintain the computing equipment in continuous operation, the automatic transfer switch may need to make the transfer to secondary power system rapidly (for example, within about 16 milliseconds).
Many power systems supply alternating current to electrical systems (for example, 60 cycles per second in North America and 50 cycles per second in many other parts of the world). During power switching in an automatic transfer switch, mechanical relays in the automatic transfer switch can be damaged by high current inrush. The risk of damage to a mechanical relay from current inrush can be reduced by controlling the switching to occur at the “zero crossing” of the alternating current of the secondary power source.
In any mechanical relay, there is a delay between the time the relay is energized and the time it is closed (typically, on the order of several milliseconds). To account for this delay, some automatic transfer switches are designed to trigger the relay in advance of the expected zero crossing by a predetermined lead time. Nevertheless, due to manufacturing differences and other differences between relays, and differences in characteristics and operating conditions in automatic transfer switches, switching in these automatic transfer switches may deviate substantially from the zero crossing. Such deviations may cause a failure of the automatic transfer switch.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to.